Make sure the fuel is getting through, check your filters (including the internal filter). You can try starting then pull the spark plugs. Place the plug with the wire attached against the engine head and crank the engine. If the spark looks slightly blue you are getting fuel, if it's yellow you're not getting fuel. If there's no spark, well, it's not the fuel - but make sure the plug is grounded properly to the engine and the wire is connected. You'll need to do this out of direct sunlight or you won't be able to see the spark properly.

While the plugs are out have a look at the color of the ceramic inside the plug. If it's black you're running too rich, if it's grey it may be too lean. Light brown/tan is the color you want. If you have feeler gauges, check the plug gap. Try replacing the plugs if they look bad. It looks like it should be .040" according to the manual for the 8/9.9 Mercury at http://www.mercurymarine.com/media/mercury/documents/0911_maintenance-8to9_9FS.pdf

Have a look at the timing belt to check for wear.

Good luck.